160 R. T. Prentki et al. 



liter"', of which 50% was ornithine, glycine and serine. The DFAA made 

 up less than 0.2% of the total DOC. Generally, the concentration of 

 DFAA over the year varied with primary production in the estuary, 

 suggesting that the amino acids originated from algal cell leakage or algal 

 cell decay. Similarly, Gardner and Lee (1975) found an average of 10 ^g C 

 liter " ' in the DFAA in Lake Mendota, a eutrophic system. 



To obtain some idea of the turnover of the labile DOC compounds by 

 the natural microflora, we used a kinetic analysis of the dark uptake of ''*C- 

 glucose, acetate, proline, and aspartate at four or five concentrations 

 (Wright and Hobbie 1966, Hobbie 1967). The equation used for the plot of 

 data was tf^ ={K,+S„) {Vmax)~^+A V„ax~^ where A is added substrate 

 concentration, /is fractional uptake of the added radioactive substrate by 

 bacteria and t is time of incubation in hours. The uptake was corrected for 

 the '''CO2 respired during incubation periods at the in situ temperature 

 (Hobbie and Crawford 1969). If the plot of if/"' vs. A was Hnear and the 

 line had a significant correlation, then the data fit a Michaelis-Menten 

 function for first order uptake. The slope of the regression was V„ai~\ the 

 Y intercept was the turnover time of the substrate at its natural 

 concentration (^^v"'), and the X intercept was an estimate of the sum of 

 the half-saturation constant (K,) and the natural substrate concentration 

 (S„) in Mg C liter"'. Unfortunately, the {Kt+S„) cannot be taken as the 

 concentration ofS„ sls K, may be equally large or even larger than the S„. 



The turnover times for these small molecular weight compounds in 

 Barrow ponds and the estimate of A^,-l-5„ were no different from those for 

 temperate lakes despite the lower pond temperature. The maximal 

 estimates of Kt+Sn ranged from 5 to 28 ng C liter ' and the turnover 

 times from 20 to 229 hours, the latter times being determined for acetate in 

 situ without adding in the '*C02 respired (hence may be an 

 overestimation) (Table 4-25). Hobbie (1967) found turnover times of 10 to 

 1000 hours over the year in Lake Erken and estimated from {K,+Sn) that 

 substrate concentrations were less than 2.4 Mg C liter " ' for glucose and 4.0 

 Mg C liter ' for acetate. In an oligotrophic marl lake. Miller (1972) 

 determined an average A^r-I-^^ to be 2.7 ^g glucose-C liter"' and 3.9 ^lg 

 acetate C liter"'. In a dystrophic kettle lake. Duck Lake, also in Central 



TABLE 4-25 Average Turnover Times (Tx) and Estimates of Maximum Natural 

 Substrate Concentrations (Kj + Sn) in Pond B, 1971 



Not corrected for CO2 respired during the incubation. 



